This invention relates to a method of treating a subterranean formation containing clay to stabilize the formation against clay-swelling and particle migration comprising: contacting said clay with an aqueous solution of a chrome complex of the Werner-type in which trivalent chromium atoms are coordinated with a carboxylic acido ligand, to form a polynucleated chrome complex on the clay surface.
Many oil bearing formations contain clays of different types and of varying amounts. Permeability losses in these formations can occur when the clays obstruct flow by swelling or dispersing. Clays may hydrate in the presence of water and obstruct flow or clay particles may migrate and obstruct flow.
Generally, the technique of water flooding which is so often used in secondary recovery processes causes swelling and dispersing of hydratable clays, such as montmorillonite. Hydration or swelling of clays is caused by the affinity of inter-layer clay surfaces and cations found on those surfaces for water. Organic cations such as amines have been used to reduce the tendency of clays to hydrate by replacing the clay cations with cations having a lower water affinity. This treatment is both an expensive and a temporary solution to the problem. After a period of time the organic cations are displaced from the clay surfaces and the clays are rendered hydratable once again.
Most clay surfaces have an inherent negative charge. Absorption of cations on the surface will neutralize the negative charge. Multilayered clay particles will tend to repel each other and disperse. However, the more firmly attached cations are to the surface, the lower the tendency of the clay particles to disperse.
Clay surfaces have been treated with multivalent cations such as calcium in order to reduce the tendency of the clay particles to disperse; however, such treatments are only a temporary remedy since the clays will revert to a sodium type when contacted with reservoir brines.
Two major and distinct problems may occur during secondary recovery processes in clay containing formations. The first occurs in secondary recovery processes in which water flooding is utilized to drive or sweep oil from an injection well to a production well. The formation around the injection well may contain swellable clays and if so, those clays can swell and obstruct the continued injection of water. The other problem results when the secondary recovery method of water flooding has been utilized. Generally, it results from the water taking the path of least resistance through the formation and forcing oil out of those channels. As the water approaches the production well, continued injection of water produces no oil since the water will tend to flow through the watered-out channels.
Several methods for treating subterranean formations containing swellable clays have been disclosed. For example, U.S. Pat. No. 3,603,396 discloses a method for increasing the permeability of a subterranean formation which, in pertinent part, comprises dehydrating at least a portion of the formation and contacting at least a portion of that dehydrated formation with a hydration stabilizer selected from the group consisting of sodium nitrate, calcium nitrate, calcium chloride, zinc chloride, sodium silicate, sodium methyl siliconate, and a vinyl ether polymer having a molecular weight from about 300,000 to about 8 million.
U.S. Pat. No. 3,603,399 discloses a method of treating a water sensitive formation by contacting it with a hydroxy-aluminum solution having a ratio of hydroxyl groups to aluminum atoms in the range of 1.5 to 2.7. Alkali or an alkaline base must be added with the aluminum compound and the two may be mixed on the surface or in the formation.
U.S. Pat. No. 2,273,040 discloses a water soluble complex compound of the Werner-type in which trivalent nuclear chromium atoms are coordinated with acyclic carboxylic acido groups having at least 10 carbon atoms. It was indicated that the compounds were useful to increase the hydrophobic properties of surfaces. In particular, it was disclosed that the compounds were useful in the treatment of clay or silicious material and in oil well treatment to render those clays or sands water repellent and more readily wettable by oils even in the presence of water.
Other patents have disclosed the treatment of formations containing swellable clays: U.S. Pat. No. 3,382,924 discloses the treatment of earthen formations comprising argillaceous materials and U.S. Pat. No. 3,621,913 concerns the treatment of clay-containing formations with an aqueous solution of aluminum salt.
In summary, many compounds including sodium, calcium, zinc, aluminum and zirconium have been described as useful in treating formations containing hydratable clays. However, these treatments have shortcomings.
For example, the compounds disclosed in U.S. Pat. No. 2,273,040 are not useful in preventing plugging at the injection well during water flooding because the fatty acid radicals of the carboxylic acido ligands are described as having at least 10 carbon atoms and such long carbon chains would render the treated surfaces oil wettable. Such a surfce would tend to restrict water injection during a subsequent water flooding treatment.
Treatments which use zirconium compounds are costly and are difficult to control since the zirconium compounds must be kept at a low pH in order to be stable in water. Aluminum compounds present the same type of pH control difficulties. In addition to closely controlling pH, when using either zirconium or aluminum compounds all the variable conditions must be closely controlled.
What is required is a method for treating formations containing hydratable clays which is efficient, economic, and useful for solving both injection well plugging problems and watered-out channel problems.